The phenomenon responsible for the operation of ultrasonic oscillator sensors is elastic wave propagation along a medium whose characteristics can be altered by a measurand. Where the characteristics of the waves propagating along the medium are dependent upon the characteristics of the propagation medium, the wave characteristics can be monitored or measured to provide an indication of the measurand value.
Many sensing applications have been found for Rayleigh or surface acoustic waves (SAWs). However, SAWs can only propagate through a semi-infinite medium, that is, a medium having a thickness which is many times their wavelength. The propagation medium used by SAW sensors is commonly a piezoelectric substrate or a piezoelectric-coated substrate, the piezoelectric material cooperating with transducing electrode structures to generate and receive the SAWs.
SAW devices are shown, for example, in U.S. Pat. No. 3,878,474 to Dias and Karrer, in which a SAW oscillator is employed as a force-sensing device, and in U.S. Pat. No. 3,786,373 to Schulz and Holland, which discloses a temperature-compensated SAW resonator device which is not specifically designed for use as a sensing element. The latter patent includes a double substrate arrangement in which interdigital electrode arrays are disposed upon a substrate which may be deposited upon the surface of a non-piezoelectric layer which, in turn, is placed upon the surface of a piezoelectric substrate giving a propagation medium that is thick relative to the wavelength of the SAWs.
U.S. Pat. No. 3,965,444 to Willingham et al. shows another temperature-compensated SAW device, having a SiO.sub.2 film layer on a substrate of piezoelectric material; and U.S. Pat. No. 4,480,209 to Okamoto, et al. shows a SAW device with a silicon substrate that is thick compared to the wavelength of the SAWs, together with a zinc oxide piezoelectric layer deposited thereon.
U.S. Pat. No. 4,456,850 to Inoue et al. shows a high-frequency piezoelectric composite "thin-film" resonator in a fundamental thickness-extensional vibration mode. It is said to have good temperature stability and resonance response. Inoue uses "thin films" having particular resonant frequency characteristics, but only as part of a thick sandwiched structure having the piezoelectric materials to achieve the temperature stability.
A number of problems arise in SAW sensing devices due to SAW characteristics or to the characteristics of the medium required for SAW propagation. One such problem is that it is difficult to operate SAW sensors while they are immersed in most liquids, a problem rendering them inappropriate for many biological and chemical sensing applications. The reason is that when SAW devices are immersed, the SAW velocity is higher than the velocity of sound waves through the liquid; a large amount of the SAW energy is therefore radiated into the liquid, and the wave is attenuated as it travels along the propagation medium.
Another problem with SAW sensors is that the thickness of the SAW propagation medium makes such devices inappropriate for certain sensor applications. Furthermore, SAW sensor devices lack the degree of sensitivity required for many possible sensor applications.
A voltage sensor which utilizes a Lamb wave delay line oscillator has been proposed by K. Toda and K. Mizutoni in the Journal of the Acoustical Society of America, Vol. 74(3), pages 677-79, 1983. The delay line uses a piezoelectric ceramic plate with a third electrode that changes the acoustic path length of the piezoelectric plate in response to an applied voltage. Although the ceramic plate is capable of supporting Lamb waves, it is still a relatively thick medium, having a thickness of 180 micrometers. This ceramic plate thickness is required for mechanical stability. However, this propagation medium thickness decreases the sensitivity of the voltage sensor and increases the velocity of flexural Lamb waves which propagate therethrough. Also the thickness required of the Toda/Mizutomi propagation medium results in a device that is ill-suited for many potential sensing applications.
A metallic Lamb-wave structure has been proposed by Uozumi et al. for use in measuring the elastic properties of thin metallic films as described in Applied Physics Letters, Vol. 43(10), pages 917-19, 1983. The Uozumi et al. Lamb-wave structure teaches a Lamb wave propagation medium that includes a metal base material on which is formed a piezoelectric film. The metallic base layer is formed by plating copper, to a thickness of three microns, onto an evaporated copper film on a disposable substrate. The piezoelectric film is deposited on this base material and transducer electrodes are deposited on the piezoelectric film to form a delay line structure.
The structure shown by Uozumi et al. is difficult to fabricate and the resulting propagation medium is wrinkled out of its plane due to the compressive stresses developed on the metallic base layer during piezoelectric film deposition that deform the metallic layer substantially. This is in contrast to the Lamb-wave sensor of the present invention, which has a propagation medium that is planar in form, even on its small scale. Also, propagation media pursuant to the teachings of Uozumi et al. are inappropriate for many possible sensor applications due to the properties of the metallic base material.
It is therefore an object of the invention to provide an ultrasonic sensor which exhibits high sensitivity.
Another object is to provide an ultrasonic sensor having sensitivity at least an order of magnitude greater than the best SAW device currently available.
Another object is to provide an ultrasonic sensor having a small heat capacity so that it can respond rapidly to heating.
A further object of the invention is to provide an ultrasonic sensor device that can operate fully satisfactorily while immersed in fluids of most types.
These and other objects, advantages, and features of the invention will be apparent from the following summary of the invention and description of preferred embodiments, considered along with the accompanying drawings.